1. Field of the Invention
The present invention relates to the preparation of structural foams and is particularly concerned with regulating or controlling the cellular structure of such foams made from foamable resin blends of polyphenylene oxide and polystyrene.
2. Prior Art
The preparation of various structural foamed articles has been described in prior patents and technical literature. Such foamed products are obtained by dispersing an inert gas through the polymer melt subjected to molding or by preblending the resin with a chemical blowing agent which releases inert gas on heating during the process. In the mold cavity the gas expands within the material constituting the polymer charge, thereby filling the mold and creating the internal cellular structure. As a result of the molding process, a surface skin is formed on the rigid foamed core.
Among the known processes for the production of structural foam resins, are those described in U.S. Pat. Nos. 3,268,638 and 3,436,446, and by Harris, W. D. in Plastics Engineering, May 1976, pp 26-30.
Methods for the preparation of polyphenylene ethers (also called polyphenylene oxides), are described in U.S. Pat. Nos. 3,306,874 and 3,306,875. The modification of polyphenylene ether resins by blending with polystyrene is described in U.S. Pat. No. 3,383,435 and elsewhere. High impact-resistant thermoplastic compositions comprising blends of polyphenylene ethers with polystyrene and with elastomer-modified polystyrene are described in published German patent applications No. 2,246,713 and 2,136,838.
Several blends of polyphenylene oxide/polystyrene are available commercially, principally under the trademark "Noryl" (General Electric Co.). Among these commercially available products are blends particularly suitable for the molding of structural foams. These are described in trade releases and in technical literature (J. L. Throne, Journal Cellular Plastics, September 1976, pp 264-283, particularly at pages 265, 272 and 273).
The blowing agent of the inert gas type most generally employed for structural foam resins is nitrogen. Among the chemical blowing agents for polyphenylene oxide/polystyrene blends, the recommended agents include: azobisformamides, p-toluene sulfonyl semicarbazides, trihydrazino-S-triazine, 5-phenyl tetrazole.
Among the problems encountered in certain structural foams, also occurring in blown polyphenylene ether/polystyrene blends, are the coarseness and irregularity of the cellular structure, which have an adverse effect on mechanical properties as well as on surface appearance. By utilization of the present invention products of improved cell structure are obtained from blowable polyphenylene/polystyrene resin blends.